Robert Hébras stepped carefully through the crumbled ruins of the village where he once lived. "There's the school bell still hanging up there, reminding me how I was always late," said the 88-year-old former mechanic.

Almost 70 years after this idyllic rural village near Limoges burnt down, there are still traces of life. Not far from Hébras's old house, the carcass of the mayor's Peugeot 202 is still parked. "When I come here, I see faces, people, not ghosts," he said. But for the French state, this is Europe's most important ghost village and there are fears that its ghosts are under threat.

Oradour-sur-Glane is unique in Europe: a fully preserved, ruined village that was the site of the worst Nazi massacre of civilians carried out on French soil. Six hundred and 42 people, including 247 children, were shot or burnt alive on 10 June 1944 in an unexplained act of barbarity. Hébras, who hid under a pile of dead bodies, was one of only a handful of survivors. He lost his mother and two sisters in the carnage during which virtually all the villagers were killed, shot or burned alive.

Unlike other Nazi village massacre sites, such as Lidice in the Czech Republic, which were razed or rebuilt and marked by monuments or fields of roses, the charred remains of Oradour-sur-Glane are the only ones to have been left untouched and still standing after Charles de Gaulle ordered they should forever bear witness. About 300,000 visitors and tourists come here each year, most walking through with horrified stares.

Robert Hébras, survivor of the Oradour-sur-Glane massacre, said it was important to preserve the ruins of the village. Photograph: Jean-Pierre Muller/AFP/Getty Images

On Wednesday, the German president, Joachim Gauck, will arrive to survey the ruins accompanied by François Hollande in a historic first visit by a German leader. But behind the pomp there is a new battle for Oradour-sur-Glane: the race to ensure the ruins stay up. The village's burnt-out shell is slowly crumbling away, eroded by time and weather, panicking French officials committed to keeping the memory alive.

In his town hall office in the new village, built after the war eerily close to the ruins, the mayor, Raymond Frugier, sat surrounded by pictures, etchings and plaques dedicated to the village's tragic past. "We're nearly 70 years on and it's as if the massacre happened yesterday. There's a sense that justice was never done and it is still an open wound," he said.

Frugier was four when his father saw the Waffen SS column approaching and took the children to hide in the forest. "The problem is that time takes its toll," he said, explaining why he has publiclyraised the alarm on the impact of the weather crumbling the walls of the ruins.

"There's a real need to keep these ruins standing for future generations. They haven't lost their authenticity. They still serve to show where certain criminal ideologies can lead, what humans can do to fellow humans."

Since Frugier raised the alarm and called for a state plan to shore up the ruins for the next 50 years, he has received scores of letters from the public offering cash. But the French state is in charge of paying for conservation of the ruins, which are classed as a historic monument and make up one of the most visited memorial centres in the country.

Each year, the government contributes about €150,000 (£127,000) to the conservation of the ruins. Ministers have promised not to abandon the village and ensure it stays standing. A culture ministry report is to be published in the coming weeks setting out what needs to be done in the long term. As France prepares for the vast centenary commemorations next year of the first world war, remembrance tourism and war commemoration are at the forefront of culture planning.

In the village, the preservation of the ruins is seen as crucial if any light is ever to be shed on the massacre. It is not clear why the SS chose to butcher all civilians: the village was not a centre of Resistance fighters, nor was it a reprisal attack. "Many villagers had never seen a German before the massacre," one resident said.

Because of the fires, only a tiny fraction of the bodies were able to be identified. Charred dolls' prams were a reminder of the children killed. This year, a war crimes prosecutor in Dortmund reopened an investigation after information found in Stasi secret police files in former East Germany led to six possible soldier suspects, now in their 80s.

Claude Milord, head of the association of families of the martyrs in the village, whose mother lost her 10-year-old sister when schoolchildren were rounded up to be killed, said it was important to keep the ruins standing to avoid any form of revisionism of the war crimes, or rewriting of history: "These ruins are unique and we have a duty of memory never to forget. For the families who lost generations of loved ones, it's like a sanctuary. It's all they've got."

As Hébras pointed out the barnyard where he fled the massacre after falling under a pile of dead and dying men, tourists gathered round him. "It's unthinkable," gasped a couple of pensioners from Tarn in south-west France.

"It's always difficult for me to come here," Hébras said. "I relive my village in my head, hear its old sounds, put faces to the ruins. But it's important to preserve these ruins and keep telling the story so it can continue to be passed down when we're no longer here."